1. Field of the Invention
The present invention relates to an alignment method in interference measurements using a hologram, and an interferometer using this method. More particularly, the present invention relates to a light interference measurement method using a computer-generated hologram in which, when the surface shape of an aspherical mirror or the like is measured using an interferometer, the object and a hologram that is disposed in the optical path of the illuminating light beam directed onto the object are set in a predetermined positional relationship, and an interferometer using this method.
2. Description of the Prior Art
In recent years, methods have become known in which aspherical surfaces are formed on predetermined surfaces of optical members in order to make it possible to obtain a good optical performance using even a small number of optical members. In the case of large mirrors and the like as well, mirrors in which the reflective surface is formed as an aspherical surface have begun to be used.
Interferometers using computer-generated holograms are known as means for performing surface shape measurements on such aspherical surfaces with a high degree of accuracy (for example, see Japanese Unexamined Patent Publication No. HEI 8-110214).
Furthermore, an interferometer constructed as shown in FIG. 4 in order to perform surface shape measurements on aspherical mirrors is known as another interferometer using such a computer-generated hologram.
Specifically, this device 140 is formed overall as a Mach-Zender type interferometer; diffused light output from a coherent light source (laser light source) 101 is converted into parallel light by a collimator 102, and is split into two light beams by a half-mirror 103. One light beam is reflected by the reflective surface (reference surface) of a mirror 104 and is used as reference light. The other light beam is reflected by a mirror 105, and is directed onto the sample surface 108a of the measurement mirror 108 via a half-mirror 106 and CGH (computer-generated hologram) 107. This light beam is reflected by the sample surface 108a, and is used as object light.
The abovementioned second light beam is constructed so that this beam is directed substantially perpendicularly onto the various parts of the sample surface 108a by the CGH (computer-generated hologram) 107. Accordingly, the object light that is reflected by this sample surface 108a advances over the incident light path in substantially the opposite direction, thus reaching the CGH (computer-generated hologram) 107 and then the half-mirror 106. However, if the sample surface 108a deviates from the ideal design shape, this surface has a wave surface shape corresponding to the amount of this deviation.
Accordingly, the reference light from the abovementioned mirror 104 that passes through this half-mirror 106 and the object light from the abovementioned sample surface 108a reflected by this half-mirror 106 interfere with each other, and this interference light passes through an image focusing lens 109 and the central through-hole 110a of a filter 110 that removes unnecessary diffracted light and the like, and forms interference fringes corresponding to the surface shape of the abovementioned sample surface 108a on the imaging plane 111a of a CCD camera 111.
Subsequently, the abovementioned interference fringe image information obtained by the CCD camera 111 is sent to an image analysis part (not shown in the figures), and the surface shape of the abovementioned sample surface 108a is analyzed by this image analysis part. Furthermore, in order to facilitate the automation of this analysis processing, a piezo-actuator 112 that is used to perform a fringe scan is attached to the mirror 104 that produces the abovementioned reference light.